The present invention relates generally to wireless communication devices and, more particularly, relates to adaptive image rejection.
Wireless communication systems are an integral component of the ongoing technology revolution. In fact, mobile radio communication systems, such as cellular telephone systems, are evolving at an exponential rate. In a cellular system, a coverage area is divided into a plurality of xe2x80x9ccells.xe2x80x9d A cell is the coverage area of a base station or transmitter. Low power transmitters are utilized, so that frequencies used in one cell can also be used in cells that are sufficiently distant to avoid interference. Hence, a cellular telephone user, whether mired in traffic gridlock or attending a meeting, can transmit and receive phone calls so long as the user is within a xe2x80x9ccellxe2x80x9d served by a base station.
One implementation of a cellular network 100 is depicted in block form in FIG. 1. The network 100 is divided into four interconnected components or subsystems: a Mobile Station (MS) 106, a Base Station Subsystem (BSS) 102, a Network Switching Subsystem (NSS) 104, and an Operation Support Subsystem (OSS) 118. Generally, MS 106 is the mobile equipment or phone carried by the user. And BSS 102 interfaces with multiple mobiles to manage the radio transmission paths between the mobiles and network subsystem. In turn, NSS104 manages system-switching functions and facilitates communications with other networks such as the PSTN and the ISDN. The OSS 118 facilitates operation and maintenance of the network.
MS""s 106 communicate with BSS 102 across a standardized radio air interface 108. BSS 102 is comprised of multiple base transceiver stations (BTS) 110 and base station controllers (BSC) 114. A BTS 110 is usually in the center of a cell and consists of one or more radio transceivers with an antenna. It establishes radio links and handles radio communications over the air interface with MS""s 106 within the cell. The transmitting power of the transceiver defines the size of the cell. Each BSC 102 manages transceivers. The total number of transceivers per a particular controller could be in the hundreds. The transceiver-controller communication is over a standardized xe2x80x9cAbisxe2x80x9d interface 112. BSC 102 allocates and manages radio channels and controls handovers of calls between its transceivers.
BSC 102, in turn, communicate with NSS 104 over a standardized interface 116. For example, in a GSM system, which will be discussed infra, the interface uses an SS7 protocol and allows use of base stations and switching equipment made by different manufacturers. A Mobile Switching Center (MSC) 122 is the primary component of NSS 104. MSC 122 manages communications between mobile subscribers and between mobile subscribers and public networks 130. Examples of public networks 130 that the mobile switching center may interface with include Integrated Services Digital Network (ISDN) 132, Public Switched Telephone Network (PSTN) 134, Public Land Mobile Network (PLMN) 136, and Packet Switched Public Data Network (PSPDN) 138.
MSC 122 typically will interface with several databases to manage communication and switching functions. For example, MSC 122 may interface with Home Location Register (HLR) 124 that contains details on each subscriber residing within the area served by the mobile switching center. There may also be a Visitor Location Register (VLR) 126 that temporarily stores data about roaming subscribers within a coverage area of a particular mobile switching center. An Equipment Identity Register (EIR) 120 that contains a list of mobile equipment may also be included. Further, equipment that has been reported as lost or stolen may be stored on a separate list of invalid equipment that allows identification of subscribers attempting to use such equipment. Finally, there may be an Authorization Center (AuC) 128 that stores authentication and encryption data and parameters that verify a subscriber""s identity.
There are several technologies in use today for different implementations of cellular network 100. When wireless telecommunications began in North America back in the 1950""s, an analogue standard called Advanced Mobile Phone Service (AMPS) was used. AMPS operated in the frequency spectrum from 824 to 894 MHz. This spectrum was then divided into 30 kHz channels for use by MS""s 106 within cellular network 100. In order to allow full duplex operation, a 30 Khz channel is reserved for each MS 106 to transmit on, and a 30 kHz channel is reserved for each MS 106 to receive on. These two channels are separated within the frequency spectrum by 45 MHz. Thus, a MS 106 transmitting on a channel at 831.21 MHz would receive at 876.21 MHz.
Dividing the frequency spectrum into multiple equally spaced channels is called Frequency Division Multiple Access (FDMA) and is illustrated in FIG. 2A. As can be seen, there is a limited number of channels 202 that can be used within the fixed frequency spectrum from 824 to 894 MHz. As a result, new technologies were developed in order to increase the capacity (number of channels) that could be supported by a cellular network 100. The first of these technologies was called Narrowband Advanced Mobile Phone Service (NAMPS). The key difference between NAMPS and AMPS is the use of a 10 Khz channel in the former. Thus, the capacity in an NAMPS system is three times the capacity of an AMPS system.
Eventually, digital technologies evolved to address the capacity issue and to improve the quality and functionality of the services provided by cellular network 100. The major difference between digital and analogue is the method used to transmit data between MS 106 and BSS 102. In an analogue scheme, the information is encoded as proportional variations in a frequency modulation (FM). In a digital scheme, the information is first digitized and then encoded using various complex modulation schemes. The modulated signal is then transmitted to BSS 102. Additionally, as a result of the digital schemes and the enhanced features they enable, the frequency spectrum from 1.85 GHz to 1.99 GHz has been allocated for new cellular type services called Personal Communications Service (PCS).
The primary digital technologies used in North American are Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA). There are several TDMA technologies currently available in the United States. One is the North American-TDMA system (NA-TDMA), also known as Digital-AMPS (D-AMPS). TDMA employs time slots to put multiple calls on the same channel. As illustrated in FIG. 2B, NA-TDMA uses the same channel scheme as AMPS; however, each channel is divided into six time slots 204a-204f. Each slot is then assigned to a different user, thus the capacity of a NA-TDMA system is six times the capacity of an AMPS system and twice the capacity of an NAMPS system. Before 1995, NA-TDMA was governed by the IS-54 standard. IS-54 is being replaced, however, by IS-136, which incorporates implementation in the PCS band, a new Digital Control Channel (DCCH), and new user services.
Another TDMA system that developed in Europe, where a similar transition from analog to digital technologies took place, is the GSM system. GSM has been adopted for use in the United States as PCS1900, which is now offered in the PCS band.
CDMA, on the other hand, is a completely different type of multiple access scheme. In CDMA, channels are not allocated by dividing the spectrum in frequency or time. Instead, a 1.25 MHz channel is used for all users within a cell. The transmission signal is prepared by first digitizing the data and then multiplying the digitized data by a wide-bandwidth pseudo noise code (pn)-sequence. Thus, as illustrated in FIG. 2C, each transmission 206a, 206b, 206c, and 206d appears as noise to all other transmissions. In order to recover the signal at a receiver, each user is given a specific (pn)-sequence that is recognized by that user""s MS 106 and BSS 102. Therefore, only transmissions coded using the specific (pn)-sequence are recognized and the rest of the transmissions are regarded as noise.
Regardless of the technology used, wireless handsets necessarily must extract all information received through a wireless communication link. In a wireless handset, a downconversion receiver is used because of the narrow channel bandwidth in relation to the receive center frequency. A downconversion receiver incorporates at least one mixer to downconvert a received radio frequency (RF) signal. Often, receivers are configured as dual conversion receivers where two downconversion stages are used, and at least two mixers are required. One problem that is encountered in downconversion receiver is the existence of an image signal that is a replica of the received RF signal. During downconversion, both the signal and the image are down converted to the same center frequency. Therefore, any noise or interfering signal contained in the image band will also appear in the downconverted signal band. This will decrease the downconverted signal-to-noise ratio, and reduce the probability that all the information received through the wireless communication link will be extracted.
The presence of an interfering image signal within a downconversion receiver must be dealt with for effective receiver designs. Techniques such as filtering may not provide the required attenuation. Therefore, the present invention is directed toward an apparatus and method for adaptive image rejection. Adaptive image rejection is provided by an adaptive image rejection circuit comprising a first Local Oscillator (LO) operating at a first center frequency and a first mixing stage, with an input coupled to an output of the first LO. The first mixing stage is used for mixing a received RF signal down to a received IF signal. The circuit also includes a second mixing stage for mixing the received IF signal down to a baseband signal and an analog-to-digital (ADC) converter for converting the baseband signal to a digital signal. In one embodiment, the ADC samples the baseband signal at 4 samples per second and inphase digital data is taken directly from the even samples, while quadrature digital data is taken directly from the odd samples.
The adaptive image rejection circuit further includes a bit error rate estimator for estimating the Bit Error Rate (BER) of the digital signal, and a frequency selector for selecting a second center frequency for the operation of the first LO when the BER exceeds a BER threshold. The adaptive selection of the center frequency based on the BER rate allows the circuit to maintain a lower overall BER by reducing the effect of the interfering image signal. In one embodiment, the first center frequency is offset by an intermediate frequency value below a center frequency of the RF signal and the second center frequency is offset by the intermediate frequency value above the center frequency. In a second embodiment, the first center frequency is offset by an intermediate frequency value above a center frequency of the RF signal and the second center frequency is offset by the intermediate frequency value below the center frequency.
There is also provided a wireless communications receiver that incorporates an adaptive image rejection circuit, and a method of adaptive image rejection. Further embodiments and implementations of the invention are also disclosed and are explained in detail below.